In a 30gallon I would probably stick with one species. If you are like me you probably want a variety of color, which makes the P. saulosi a prime candidate. They are dimorphic, with males being blue with black bars while females are a nice yellow. They are also a smaller species, maxing at about 4". The only thing is that they can be aggressive so you will want a good ratio of males to females. I would suggest going for 2 males and 5 or 6 females. You should always try for a 1M:3F ratio with any mbuna species. Also make sure to have lots of rock work and hiding places for the non-dominant to hide.
One other thing, when P. saulosi are young they are all yellow. As the males mature the turn blue, but holding females can sometimes display male colors as well. Here is a website that can give some information on this species, as well as others.
For the P. saulosi:
http://cichlid-forum.com/profiles/species.php?id=1
For other species:
http://cichlid-forum.com/profiles/
).